Bingo, that was the problem in my setup: as the patch was applied through a script with others, I had missed the error message about the conflict (I have also another conflict which can be safely ignored so the new one did not catch my eye)... So the patch was only half-applied, and the final code is broken.
How did you solve the conflict (I am not a scheduler expert)? I can retry running the patched kernel with your resolution, to check if everything is ok.
After looking a bit more, the conflict resolution seemed straighforward, so I gave it a go. The attached version booted fine, so the initial problem was purely PEBCAK... Sorry for the noise!